Fire resistant electrically conductive rubber article



Feb. 8,1944. D. BULGIN 2,341,350

FIRE RESISTANT ELECTRICALLY CONDUCTIVE RUBBER ARTICLE Filed oct. 31, 1958 f fief y,

Home Refor'oan Rubbef' 11j-fa ..2

,l INVENTOR. .DOUGLAJUL GNV.

ZTTORNEY um@ annahm..

- yUNITED s'rATes ieri'txv'r carica mns assrs'rAN'r moraromr ooNnoo'rrvn nonnen ARTICLE Douglas Bulgin, Erdington, Birmingham, England, assignor to Dunlop Tire and Rubber Corporation, Buialo, N. Y., a corporation of New York 'Application october si, iss, serial No. 237,943

^ In Great Britain November-is, 193':-

'z claims. (ci. 15e-.43) I name smother-'ing gases. such as carbon dioxide,

My invention relates to improvements in or relating to the production of electrically. conductive and rire or flame resistant articles com; waterbouw r maielmbber nlileieesush related. :n nirfrils asl itt'erhT-balata and synthetic.l rubbers suchA aseopr'eiifw`m In certain uses, suc'nas in the perating'rooms of hospitals, it is very desirable to use rubberfor ooring and for parts of the anaesthetising apparatus which is suiliciently conductive electrically to' prev'nt the 'accumulation of static charges of electricity, and the discharge of which might cause the ignition of vapors used to pro-V duce anaesthesia. In the use of certain `high voltage apparatus, for example, that used in diathermic treatments, there is also danger due at or above the ignition temperature either with or without heat absorbing. action. A number of such materials may be incorporated in the unto the voltage which may'caus'e the passage of a. current, through the conductive rubber, oi suilicient intensity. to cause ignition. Articles of electrically conductive rubber when exposed to electric heating effects such as the above are liable to ignite and, when ignited, to continue to burn even though the cause of the ignition should terminate. This iis-particularly the case when the rubber is in the form of thin articles. such as conductive rubber sheets and tubes to which air is freely accessible.

In my .present invention the danger of coru-V bustion and flame propagation in articles of the labove type is obviated, or minimized, and articles of rubber having the required conductivity, par-l ticularly at the surface, and having a name retardant or quenching action are provided.

In my invention lfire resistant, electrically conductive articles o rubber are formed with a surface. layer of electrically conductive rubber or rubber-like material and a supporting layer of rubber composition which is electrically 'nonconductive, or relatively so, and which of a thiclmess equal to, or exceeding, that of the conductive surface layer. The conductive surface layer may be provided with a relatively large proportion of conductive ingredients, genorally solid, such as conductive forms of carbon. It may contain also some llame retardant ma- -terials, but only a relatively small amount of such materials can be incorporated without reducingrthe rubber content excessively. In the underlying layer, however. in which there need be little or no conductive material, there is incorporated a large quantity of materials that resist or inhibit propagation of llame. These materials may act to absorb heat rapidly and thus to reduce the temperature below the ignition point, or may be oi such nature as to evolve derlying flame resistant or.repellent layer.

In the accompanying drawing which illustrates diagrammatically embodimentsoi the invention, Fig. 1 is a cross section of a sheet embodying two layers, one being iiame resistant and the other conductive. section of a sheet containing a i'lame resistant layer covered on opposite faces with conductive surfacelayers.

In the embodiment shown in Fig. l, a layer l0 of name retardant rubber, for example, a rubber.A mix containing incombustible flame smothering materials or materials that evolve incombustible smothering gases, or a mixture of such mate-` rials, is covered with a layer Il which is rendered conductive by being' compounded withcon- Y ductive materials. g

As an illustration of suitable compositions for.. the conductive layer. a mix of the following compositions may be employed:

Rubber 60.5 Sulphur 2,25 Zinc mde 4.5 Stearic acid 2.0 Parailin wax 2.0 Antioxidant y 9.5 f Mercaptobenzthiazole 0.75 Acetylene black. 28.0-

The nature and functions of the variousingreclients in the above mix are commonly understood in the rubber art and the usual equivalents may, therefore, be substituted. Also thev class of conductive elements or materials are generally known, and those commonly used and having no deleterious action on the rubber may be employed.

The above mix is calendered into sheets and is applied in an unvulcanized state to unvulcanized sheets calendered from a mix of the following composition:

In the above composition, zinc oxide, China c lg and asbestine are incombustible substances which Fig. 2 is s *are* Ereadily absorb heat generatedvbywcgmbustignwand I' fnd 5 reduce the temperature belotvthe igiiitin tc'praturen'the be noted that these materials exceed over 50% by weight of the composition.

The composite sheet thus formed is then vulcanized for 30 minutes at 150 pounds per square inch of steam pressure, and the vulcanized sheet may be used as surface material for iioora. When used as ilooring, a. suitable thickness for the conductive layer is about 3 millimeters applied to a non-conductive layer of the same or greater thickness. For use with apparatus, the unvulcanized composite sheet may be formed info a tube, .or into other shapes or articles, and subsequently vulcanized. When used for tubing, the conductive layer may be about 0.25 millimeter in thickness and be applied to a non-conductive layer of the order of 1.25 millimeters in thickness. Substances .cambleszinsratineihere ...resislariirrgnertiesimbbe uw .be added?. .the above compositionsuprim;t;yilfiiati.MEX amplsbi' such substances are selenium, chlgriny atebbeLor chlordhdibbgrbtsr mixnsium carbonate and ammonium phosphate.

ascisse of the non-conductive sheet before or after vuicanization of the latter, with or without subsequent vulcanization of the conductive layer so applied.

5 A sheet solely of electrically conductive rubber pf 2 millimeters in thicmesa when exposed to a i high voltage electric arc caught are in seconds and continued to burn. In contrast, a composite sheet of the same thickness oi the present i0 invention consisting ot a conductive layer 0.35 millimeter thick, and a non-conductive layer 1.55 millimeters thick, caught fire only after being subjected to the same arc for 150 seconds and did not continue to burn upon removal ot the arc.

What I claim is:

1. A i'lre resistant electrically conductive vulcanized rubber having a surface layer of electrically conductive. rubber composition, and an underlying rubber composition containing incom- 20 bustible.- flame smothering material.

2. A nre resistant electrically conductive vulcanized rubber article having a surface layer of electrically conductive rubber composition, and an underlying layer of rubber composition ccntaining materialevolving incombustible, 11eme smothering gases above the vulcanizing temperature and below the ignition temperature oi rubber. f

3.*The article of claim 2' evolving material is a. carbonate.

4. The article of claim 2 ln which said gas evolving material is an ammonium compound.

5. The article oi claim 1 in which said incombustible flame smothering material is a mineral.

6. A lire resistant electrically conductive vul canized rubber having a surface layer of electrically conductive rubber composition and an underlying rubber composition containing an incombustible, name smothering borate.

canized rubber having a. surface layer of electrically conductive rubber composition and an underlying rubber composition containing over 50 per cent by weight of incombustible. name smothering materia.

DOUGLAS BULGIN.

in which said gas' 7. A lire resistant electrically conductive vul- 

